The uneven distribution of species richness is a fundamental and unexplained pattern of vertebrate biodiversity. Although species richness in groups like mammals, birds, or teleost fishes is often attributed to accelerated cladogenesis, we lack a quantitative conceptual framework for identifying and comparing the exceptional changes of tempo in vertebrate evolutionary history. We develop MEDUSA, a stepwise approach based upon the Akaike information criterion for detecting multiple shifts in birth and death rates on an incompletely resolved phylogeny. We apply MEDUSA incompletely to a diversity tree summarizing both evolutionary relationships and species richness of 44 major clades of jawed vertebrates. We identify 9 major changes in the tempo of gnathostome diversification; the most significant of these lies at the base of a clade that includes most of the coral-reef associated fishes as well as cichlids and perches. Rate increases also underlie several well recognized tetrapod radiations, including most modern birds, lizards and snakes, ostariophysan fishes, and most eutherian mammals. In addition, we find that large sections of the vertebrate tree exhibit nearly equal rates of origination and extinction, providing some of the first evidence from molecular data for the importance of faunal turnover in shaping biodiversity. Together, these results reveal living vertebrate biodiversity to be the product of volatile turnover punctuated by 6 accelerations responsible for >85% of all species as well as 3 slowdowns that have produced ''living fossils.'' In addition, by revealing the timing of the exceptional pulses of vertebrate diversification as well as the clades that experience them, our diversity tree provides a framework for evaluating particular causal hypotheses of vertebrate radiations.evolutionary radiation ͉ macroevolution ͉ phylogeny T he extremes of vertebrate richness have long fascinated evolutionary biologists (1, 2). Species richness of some groups, like teleosts, ostariophysans, birds, mammals, and frogs, is often attributed to accelerated diversification accompanying ecological adaptive radiation or the acquisition of key innovations (3). In contrast, the evolutionary stasis exhibited by many representatives of the sparest branches, such as tuataras, coelacanths, and the bowfin, has been attributed in part to historically low rates of cladogenesis (4, 5). However, the hypothesis that differential diversification rates explain vertebrate biodiversity has rarely been tested. It is possible that, with regard to species richness, some or many of the classic vertebrate radiations might not be exceptional at all. This is because simple models of lineage origination and extinction are expected to produce clades of varying sizes (6). Testing hypotheses about diversification rate at broad phylogenetic scales is challenging for at least 2 reasons. First, most comparative studies of diversity focus on patterns within major clades rather than across them (7-9). Second, most current diversification methods perf...